What can nurse practitioners do?

Depends on state law: The scope of practice for arnp's depends on the state where they practice, and they generally have to work in coordination with an md. Here in washington state, they are under the purview of the nursing board, not the medical board so that provides a degree of flexibility. I work with several very talented nurse practitioners and they fill a valuable role.

Most things Drs. do!: As Dr. Baxter says, this depends on state laws. Generally they require some supervision by a doctor, but they can see patients on their own and prescribe many medicines on their own (though need a supervising doctor's approval for controlled substances). Most nurse practitioners work in primary care and many specialize in women's health.